The aim of this research is to understand the mechanism by which poliovirus infection causes a specific inhibition of host cell protein synthesis, while production of viral proteins proceeds efficiently. Extracts derived from infected cells are unable to translate capped mRNAs in vitro, although they are capable of translating viral RNA, which is not capped. This restriction against translation of capped mRNAs is because of a virus- induced inactivation of a protein synthesis initiation factor (eIF-4F), which is required for the binding of capped mRNAs to the ribosome. The initiation factor contains a small polypeptide subunit which binds cap structures, and a large subunit, which becomes cleaved shortly after poliovirus infection. Cleavage of this large subunit presumably inactivates the function of the factor. The viral gene responsible for cleavage of the initiation factor subunit is a gene that codes for a protease. This protease, however, does not catalyze the cleavage directly, but rather it activates a latent, cellular protease which then cleaves the initiation factor. Studies proposed in this application will identify the latent, cellular protease and define the biochemical mechanism of the protease cascade. An additional objective is to determine how poliovirus RNA binds to ribosomes and initiates translation in the absence of a capped 5' end and therefore without the aid of the cap recognition system. Experiments are proposed to identify the sequences and/or structures in the 5' noncoding region of the viral RNA that are required for cap-independent ribosome binding, and also to isolate trans-acting factors which interact with this region. These studies will contribute to our understanding of how protein synthesis is regulated in infected cells, in ways that determine virus-host cell interactions which likely contribute to neurovirulence and host range properties of this virus.